Device-to-device, D2D, communications in cellular spectrum is a relatively new concept that targets scenarios in which communicating parties are in the close proximity of each other, see G. Fodor et al, “Design Aspects of Network Assisted Device-to-Device Communications”, IEEE Communications Magazine, March 2012. An advantage with using D2D communication is that the capacity of a radio communication network as a whole is increased without a corresponding impact on cellular infrastructure. Thus, the infrastructure radio communication network may be offloaded in terms of traffic between wireless devices.
Moreover, D2D communication enables infrastructure-less communication between wireless devices. This may be of importance in, for example, emergency, national security and public safety situations, since during these situations load on the radio communication network(s) is generally high. Furthermore, an emergency situation may for example occur where only limited coverage by the radio communication system is provided. In such a situation, D2D communication may improve coverage by allowing wireless devices within an area to communicate with each other. In addition, local communication between wireless devices using D2D communication is achievable without a need for radio coverage by the radio communication system or in general, the radio coverage of a cellular infrastructure independently whether the infrastructure comprises one radio access technology (RAT) or a plurality of RATs.
Some level of synchronisation is required between a transmitter and a receiver in D2D communication, e.g. between wireless devices. The level of synchronisation required varies with the type of service intended to be pursued in the D2D communication. However, even though supporting direct wireless device discovery and/or communication requires significant shift from the existing LTE specifications, it is desirable to minimise such changes. One of key aspects of LTE cellular communications is that they operate in a synchronised fashion (with the exception of some random access and cell search procedures). In other words, the wireless device is expected to acquire local synchronisation to a serving cell by tracking reference signals suitable for such purpose (e.g. PSS, SSS, CRS).
Therefore, in order to reuse some of the existing LTE procedures, it is desirable to also let direct discovery/communication procedures operate in a synchronous fashion. Furthermore, some studies showed increased performance with synchronised direct mode operations, mainly because interference can be coordinated more efficiently than in an asynchronous network. In any case, for any radio communication link, synchronisation is required for enabling a receiver to decode information content transmitted by a transmitter.
Network assisted device-to-device communication is well known in the art, wherein methods have been developed, wherein the network assists in the pairing and synchronisation of candidates for device-to-device communication. However, in order to fully achieve the advantages of off-loading the infrastructure of a cellular radio network, direct discovery enabling autonomous establishment of device-to-device communication is desired.
Direct discovery could be achieved by letting wireless devices periodically transmit discovery signals, also known as beacons. Devices in close proximity of the transmitting wireless devices monitor the beacons and detect devices to which device-to-device communication may be established by means of information carried by the beacons. Typically, beacons carry data enabling the identification of the transmitting wireless devices.
In order to allow their efficient coherent demodulation, device-to-device control messages e.g. beacons, are typically associated to reference signals transmitted to enable estimation of the sequence associated to the beacon. Arbitrary mappings of the direct control reference and messages to the sub-frame are possible. However, a regular pattern is generally used. Even though these direct control reference signals only occupy a limited amount of resources, the periodicity of these signals implies that they generate an foreseeable resource and energy cost. FIG. 4a illustrates allocation of a direct control message 52 e.g. a beacon associated with a direct control reference signal 53 and in an OFDM subframe 51. The beacon typically appears periodically in the same time and frequency in the subframe.
Hence, the transmission of direct control messages and signals has a negative impact on the energy consumption in the transmitting wireless device. Thus, there is a desire to limit the amount of resources used for beacon transmissions, from a wireless device. However, in order maintain discovery latency on an acceptable level, beacon transmission must be performed with at least a minimum periodicity. For many consumer applications, it is reasonable to assume an average beacon transmission periodicity of at least hundreds of milliseconds, if not some seconds.
Furthermore, as mentioned before, synchronisation is necessary before real signal transmission and reception can be started. Considering the lack/partial network coverage, i.e. the out-of-network coverage case or asynchronous neighbouring eNBs scenario, i.e. in-network coverage case, a common sync reference might not be available. This triggers the need of a synchronisation reference signal sent by the wireless devices autonomously. FIG. 4b illustrates allocation of a reference signal, a synchronisation message and a direct control sequence in an OFDM subframe.
However, in some situations several different wireless devices within an area are transmitting synchronisation reference signals simultaneously. A wireless device who wants to establish connection with a peer will check a predefined bandwidth, and there find several different synchronisation reference signals. But the synchronisation reference doesn't contain any information to make it possible to determine which beacon it is associated with. Thus, the wireless device may need to try different possibilities before finding the correct reference.
Hence, present solutions for autonomous establishment of device-to-device communication include a discovery and synchronisation process that is typically time and energy consuming. It is an object of the present disclosure to provide solutions that improve autonomous device-to-device communication.